Transmission Control Protocol (TCP) [1,4,10], the transport layer protocol widely used in the internet has gradually evolved over the years with newer mechanisms that have helped improve its performance. TCP has been designed as a stream based reliable transfer protocol that can work on top of an underlying network protocol. The built-in end-to-end congestion control and reliable data transfer mechanisms minimize the complexity within the network and more significantly are a critical factor in the robustness of the internet.
Throughout the evolution of TCP, a loss in the network has always been an indication of congestion. This is not the case in many situations, especially when the connection spans lossy links. Due to this, TCP performance is good over wire-line internet links but degrades over lossy links [5,7], such as wireless and satellite channels. Losses due to link characteristics cause TCP to reduce its sending rate thus resulting in bandwidth under-utilization. Wireless links for example, are often characterized by high bit error rates due to channel fading, noise, interference, and intermittent connectivity due to hand-offs. TCP performance in such networks suffers from throughput degradation and very high interactive delays, because the sender misinterprets random packet losses as congestion.
Various mechanisms have been put forward to alleviate the affects of non congestive losses on TCP performance over lossy links. These mechanisms adopt a variety of schemes such as Forward Error Correction, split TCP connections, local retransmissions, link aware protocols, Explicit Loss Notification (ELN). A brief description of some mechanisms is provided.                (1) Split connection protocol: [6] This approach splits the TCP connection between the sender and the receiver into two separate connections, one between the sender and the base station and the other between the same base and the receiver. A base station is a router sitting at the boundary of a lossy network. A specialized protocol fine tuned to the link characteristics is used over the lossy hops to reduce the problems faced by standard TCP over lossy links. The major drawback of the system though, is that it violates end-to-end semantics. Acknowledgements for packets may reach the sender before the data actually reaches the receiver. This also makes the protocol vulnerable to base station crashes.        (2) Snoop Protocol: [8] Snoop protocol is basically a link aware protocol. The mechanism uses an agent sitting on the base station that monitors TCP packets in both directions and caches them. It retransmits such packets if it detects their loss either through duplicate acknowledgements or a local timeout for the acknowledgement.        (3) Explicit Loss Notification: [12] ELN is a link aware mechanism using which the cause for the loss of a packet can be communicated to the TCP sender. The ‘snoop’ agent sitting at a ‘base’ station monitors the traffic and keeps track of holes in the sequence space of every connection as it receives data segments. A hole is a missing interval in the sequence space. It then sets the ELN bit on acknowledgements, especially duplicate acknowledgements, if they correspond to a segment in the list. The source uses the ELN notification in its congestion detection mechanism. All such snoop protocols have the inherent disadvantage of being incompatible with end-to-end flows that employ IP layer security using encryption or other means.        (4) U.S. Pat. No. 5,974,028: The methodology for providing transport protocol within a communication network having a lossy link. The receiver distinguishes between packets received with non-congestion bit errors and packets not received due to congestion. It sends selective acknowledgement to the sender indicating which packets were received with non-congestion bit-errors, while suppressing the duplicate acknowledgements to avoid invocation of congestion control mechanisms.        